Conventionally, a loop detector and an ultrasonic detector are well known vehicle detectors for determining traffic flow such as the amount of traffic or the occupation rate.
A loop detector detects a vehicle by a loop coil buried underground for sensing inductance change when a vehicle passes over the coil.
An ultrasonic detector generally uses a so-called active sensor for detecting the reflection of an ultrasound transmitted from its sensor such that the detector detects the difference between the arrival time of the ultrasound reflected from the vehicle and the road, respectively.
An example of such ultrasonic detector is described in Japanese Patent Application Laid-Open No. 60-78373 (refer to Claims).
Such conventional vehicle detectors have the following drawbacks.
Since the loop detector requires the coil to be buried underground, burying work is needed, and the detector might be inapplicable to a place, such as a bridge, etc. when it is difficult to secure an installation place.
On the other hand, an ultrasonic detector has drawbacks such as                {circle around (1)} when the ultrasonic detector is equipped at the side of a road to preserve the aesthetic beauty of town, precision detection of a vehicle is difficult, and        {circle around (2)} the cost of the detector tends to be high, because of its high power consumption.        
(1) Generally, a transmitter/receiver of the ultrasonic detector must be set almost directly above a vehicle running on the road in order to enable an almost vertical transmission of ultrasound to the vehicle. Therefore, the ultrasonic detector is installed such that the transmitter/receiver fixed to an overhead horizontal bar supported by a pole set at the side of the road faces downward to the road and vehicles. The bar must be of sufficient length for the ultrasonic detector to be set directly above the center of the vehicles, which results in shortcoming that the horizontal bar supported by the pole and projected to a space above the center of the road is not only detrimental to scenic beauty of the town, but also needs high construction cost. In particular, the above described problems are remarkable in the case of installing plural vehicle detectors to cover plural traffic lanes on the road; a long overhead bar must be supported by two poles standing at both sides of the road.
To solve these problems, it is conceivable for the ultrasonic detectors to be installed a so-called “side-fire” configuration at a slanting incline position to the vehicles, in place of the position directly above the vehicles (i.e., so-called “right-above” installation), in order to eliminate or shorten the long overhead bar. However, the transmitter/receiver by the “side-fire” installation is set at a downward slant to the vehicles, and a misdetection of the vehicles may occur due to the effects of rain and wind, or multiplex reflection. Specifically, a case where an ultrasonic transducer vibrates its own vibration frequency when rain falls from a sideward direction, etc. and hits the ultrasonic transducer or a case where the detector receives a reflected multi-passed wave other than the reflected wave directly from the vehicles may be determined as “the present of a vehicle” by the sensor.
(2) Since the ultrasonic detector is an active sensor, electricity consumption tends to be high. Power for the ultrasonic detector is usually supplied through a cable.
Therefore, the ultrasonic detector becomes costly because cable connection for power supply is necessary and its high power consumption is high.